RFSim99 is freeware software that helps with impedance matching and can simulate RF circuits. Plots Amplitude and phase response, different display styles including Smith chart. Uses one and two-port Scattering parameters. You can for example import an S-parameter file of your filter from your network analyser and have the software calculate matching. Requires Windows 95, 98, NT or 2000 (WinXP virtual machine runs it just fine)

Microstrip Patch Antenna Calculator This calculator calculates the length and width of a patch antenna based on the frequency, dielectric constant of the dielectric and the height of the thickness of the patch antenna substrate.

Skin Depth Calculator Enter the frequency and the characteristics of the material to calculate the skin depth.